Microsoft confirms that Windows RT, tools, apps and Office RT need nearly half of the 32GB in its entry-level tablet

Microsoft has confirmed what many had suspected, that it didn't offer a 16GB Surface RT tablet because there would have been virtually no room for customer content on the device.

According to a FAQ published Monday, a 32GB Surface has approximately 16GB of storage space available after accounting for the Windows RT operating device, numerous built-in "Windows Store" (formerly "Metro") apps, OS recovery tools, and the bundled Office RT.

The operating system, apps and Office RT consume 8GB, while the Windows recovery tools occupy another 5GB. In other words, about 45% of the storage in an entry-level Surface RT has already been spoken for before the tablet is unboxed.

(The numbers don't add up to 32 because while drive space is almost always expressed in "digital" gigabytes calculated as 10 to the ninth power, or 1,000,000,000 bytes, most operating systems, including Windows, calculate "binary" gigabytes as 1024 cubed, or 1,073,741,824 bytes. A "digital" gigabyte then, is only 93% of a "binary" gigabyte. That's how the advertised 32GB in the Surface ends up being seen by Windows as 29.8GB.)

A 64GB Surface RT provides about 46GB of available storage space, or 78% of the total.

Although Microsoft spelled out the two storage options when it introduced the Surface last June, it did not reveal how much would be available to the customer or say why it wasn't offering a 16GB model, presumably at a lower price. Many assumed that Windows' size simply precluded a 16GB device.

It wasn't until the day of the Surface RT launch event, where prices were disclosed that someone from Microsoft first talked about how much storage space would be available to the user.

In a wide-ranging Reddit "Ask Me Anything" discussion thread on Oct. 16, Ricardo Lopez, test manager for Surface RT, said, "After the OS, Office RT and a bunch of apps, you will still have more than 20GB."

It's unclear why Lopez claimed 20GB when the actual space available is approximately 16GB. Microsoft did not reply to a request for an explanation or comment.

Unlike the iPad, the Surface RT also lets users store non-app content on MicroSD memory cards, which sell for around $30 for 32GB or $60 for 64GB, or on even cheaper USB flash drives. Microsoft also provides 7GB of cloud-based storage for free through its SkyDrive service; by default, Office RT saves its documents to SkyDrive.

One thing is clear: Microsoft's bottom line won't be affected by starting at 32GB. An analysis Monday by IHS iSuppli of the Surface RT's "bill of materials" (BOM), an accounting of the component and manufacturing costs, showed that storage space contributed just $34 to the BOM, or $17 for the additional 16GB.

By iSuppli's estimate, Microsoft will generate $315 for each 32GB Surface RT -- the list price minus the BOM -- for a margin of nearly 53%, higher even than Apple's entry-level 16GB iPad.

When is 32GB tablet not 32GB? When almost half is consumed by the OS, tools and bundled apps. (Data: Microsoft.)